[0001] The present invention relates to an improved process for the removal of pyritic sulfur
from coal.
[0002] The present energy crisis has produced both economic and governmental incentives
to use more coal as fuel to replace oil and gas imported in ever increasing amounts.
Counterbalancing these incentives are governmental regulations which establish a permissible
level of pollutants from the combustion of these fuels. One of the major pollutants
is sulfur dioxide. Unfortunately, most of the coal reserves in this country contain
sulfur in amounts which are too excessive to burn in compliance with existing law.
Major consumers of coal, such as electric utilities, have two alternatives to follow,
namely they can buy low sulfur content coal or use flue gas desulfurization to remove
sulfur dioxide after combustion. The first alternative would be most feasible if sulfur
could be removed from coal using methods which are both practical and economical.
[0003] In my copending application Serial No. 952,108, filed on October 17, 1978, entitled
"Mild Oxidative Coal Desulfurization", a process for the removal of pyritic sulfur
from coal by chemical oxidation utilizing a basic reaction medium is disclosed and
claimed. In this process, an aqueous slurry containing finely-divided coal particles
is first prepared and the pH of the slurry maintained at a value of between about
8 and 12 by the addition of an alkali or alkaline earth metal hydroxide or carbonate.
The coal slurry is then agitated while being treated with oxygen or an oxygen containing
gas such as air. Alkali and alkaline earth metal hydroxides and carbonates that are
useful in this process are the hydroxides and carbonates of sodium , lithium, potassium
and magnesium. The process can be carried out at temperatures that are only slightly
above ambient, e.g. 40-70
.C, and at atmospheric pressure. Therefore, there is no need to buy or maintain equipment
capable of handling abusive conditions.
[0004] A disadvantage of the above described process is that it requires the use of large
quantities of the alkali or alkaline earth metal hydroxide or carbonate which increases
the cost of the process. Moreover, the product coal after treatment may contain an
undesirably high quantity of the alkali or alkaline earth metal, e.g. sodium, which
could eventually lead to corrosion of the combustion equipment. The quantity of metal
in the coal can be significantly reduced by acid treatment but this also adds to the
cost of the process.
[0005] It has been discovered in accordance with the present invention that the above mentioned
disadvantages can be effectively avoided and an improved process provided for the
removal of pyritic sulfur from coal by adding lime to the reaction products in order
to regenerate alkali metal hydroxide for use in the process. The alkali metal hydroxide
is formed by reaction of the lime with the metallic sulfate that is produced during
oxidation of the pyritic sulfur in the coal. It has been found that when the regenerated
alkali metal hydroxide solution is used as the caustic in the process, the pH of the
coal slurry should be kept below about 8 and preferably at a value of about 5 or 6.
The slurry should also be maintained at a temperature of at least about 70'C. In addition
to regenerating alkali metal hydroxide for use in the process, a further advantage
of this improved process is that at the lower pH values contemplated there is less
chance of objectionable quantities of metallic impurities depositing in the treated
coal.
[0006] The present invention will now be described in greater detail with reference to the
accompanying drawing wherein:
Figure 1 is a block diagram illustrating the improved process of the present invention;
and
Figure 2 is a graph which illustrates the relationship between the pH of the slurry
and the characteristics of the product coal.
[0007] The process of pyrite removal from coal by chemical oxidation in an alkaline medium,
e.g. sodium hydroxide, proceeds according to the following equation;
(1) FeS2+ 4 NaOH + 3.75 02 → 0.5 Fe203 + 2 Na2S04 + 2 H20
It will be seen from equation (1) above that the pyritic sulfur in the coal is oxidized
to sulfate which is soluble in the reaction medium. The iron remains in the treated
coal as an insoluble oxide or hydroxide. This desulfurization process may be carried
out at high rates of pyrite removal from the coal under certain optimum conditions,
e.g. temperatures of between 50 and 60°C, pH values of about 10 or 11 and at atmospheric
pressure. Approximately 90% of the pyrite can be removed from the coal in about two
days under these conditions. Conventional solids-liquid separation techniques can
be employed to recover the treated or desulfurized solid coal.
[0008] It has been proposed to modify the desulfurization process described above by adding
lime, e.g. Ca0 or Ca(OH)
2, to the reaction product in order to convert the sodium sulfate to sodium hydroxide
and thereby to regenerate additional sodium hydroxide for use in the process. This
regeneration of NaOH proceeds according to the following equation:
The regeneration reaction as represented by equation (2) above actually does not go
to completion with all of the sodium sulfate reacting with lime to form additional
NaOH. Some unreacted sodium sulfate will remain in the liquid phase of the reaction
product along with the regenerated sodium hydroxide and a small amount of CaS0
4' Also, the solid phase in the reaction product will contain some Ca(OH)
2 along with the CaSO
4· 2H
20. It has been found that the presence of calcium ions in the regenerated NaOH solution
can dramatically impede the oxygen leaching of pyritic sulfur in the coal at pH values
of above about 8. In addition, the problem of residual sodium and calcium in the product
coal still persists.
[0009] It has been discovered in accordance with the present invention that the inhibiting
effect of calcium ions when present in the regenerated NaOH solution upon the oxygen
leaching of pyritic sulfur from coal can be effectively overcome by lowering the pH
of the coal slurry to a value of below about 8 and preferably to a pH of about 5 or
6. At these lower pH values, the reaction rate slows up considerably and oxygen leaching
of pyritic sulfur from coal, e.g. approximately 90% removal, may take as long as two
weeks to complete. However, it has been found that the reaction rate can be significantly
increased by carrying out the reaction at slightly elevated temperatures of at least
about 70
.C. Approximately 90% removal of pyritic sulfur from coal can be attained at these
temperatures over a period of about six days.
[0010] Another inherent advantage of using the lower pH is that less calcium and sodium
are incorporated into the product coal. It has been found for example that less than
about 0.1 weight percent of calcium and sodium are deposited in the coal when the
slurry is maintained at a pH of about 6.
[0011] It will be understood that the present invention is broadly applicable to the treatment
of various types of coal. In particular, the process is directed to the desulfurization
of bituminous coals which are combusted to generate steam in electric utility plants
or industrial boilers. Coals that may be treated in accordance with the present invention
are the medium and high volatile coals such as, for example, Ohio No. 6 coal. It will
also be understood that the present invention is not limited to the treatment of the
above mentioned coals alone and that coals other than bituminous coals such as anthracite
and lignite coal may be treated as well. In general, the coals that are treated in
accordance with the present invention will contain pyritic sulfur concentration in
the range of from about 0.5 to about 41 by weight of the coal.
[0012] The raw coal which is obtained from mines in chunk size, for example, is first reduced
to a finely divided particle size. The particle size of the coal should be sufficient
to expose a substantial fraction of the total surface of the pyrite that is contained
in the coal. Generally speaking, the coal is reduced to a particle size smaller than
about 200 mesh.
[0013] The finely divided coal particles are formed into an aqueous slurry, for example,
by mixing the coal particles together with water in a reactor. The coal slurry should
preferably possess a solids concentration in the range of between about 4 and 40%
by weight coal.
[0014] The desulfurization process is started by adjusting the pH of the coal slurry to
a value of below about 8 and preferably to a pH of about 5 or 6. The,pH of the slurry
is initially adjusted by the addition of a caustic, such as sodium hydroxide or other
alkali metal hydroxide as shall be described further hereinafter. The coal slurry
is then agitated and subjected to an oxidizing medium such as oxygen or an oxygen-containing
gas e.g. air. The oxygen or air should be introduced in intimate contact with the
coal slurry. This may be accomplished for example by bubbling sxygen through the slurry
or by aerating the slurry in the reactor. It may be necessary to periodically add
caustic to the slurry in order to continuously maintain the pH of the slurry within
the desired range. The slurry is also maintained at a slightly elevated temperature
of about 70°C. The pressure in the reactor is kept at about atmospheric.
[0015] Sodium hydroxide or other caustic used in the process is regenerated according to
equation (2) above by the addition of lime e.g. CaO or CaOH, to the reaction product.
The reaction product is filtered and removed from the reactor and is fed together
with the required amount of lime to a separate reactor, e.g. a caustic regeneration
reactor. The regenerated sodium hydroxide that is formed in this reactor is then filtered
and fed back to the first reactor for use in the process. The solid CaS0
4 that is also formed in this reaction is then removed from the regeneration reactor
and discarded as waste.
[0016] Although it is preferred to employ sodium hydroxide as the caustic reagent in the
practice of the present invention, it is expected that other alkali earth metal hydroxides
such as lithium and potassium hydroxide will work as well. However, both lithium and
potassium hydroxide are expensive to employ on a commercial scale and therefore the
use of these caustic materials in the process may be prohibitive. Although the alkali
and alkaline earth metal carbonates such as sodium carbonate as well as the carbonates
and hydroxides of magnesium are useful in the desulfurization process disclosed in
my copending application Serial No. 952,108, these caustic materials are not compatible
in the regeneration process and therefore should not be employed.
[0017] Referring to Figure 1 of the drawing, there is shown a block diagram of the process
wherein the reactor 10 is the main leaching reactor and reactor 12 is the caustic
regeneration reactor. As illustrated, reactor 10 has its own filter 14 and pump 16
for feeding the reaction product into the reactor 12. The reactor 12 also has its
own filter 18 and pump 20 for feeding the regenerated NaOH back to the reactor 10.
Pumps 16 and 20 are activated by a pH controller 22 which is connected to the reactor
10.
[0018] The following examples will serve to further illustrate the present invention.
EXAMPLE I
[0019] The leaching reactor used in this experiment was a 1 liter reaction kettle provided
with a gas inlet at the bottom. This reactor was also equipped with a heating mantle,
thermocouple, mechanical stirrer and a pH controller. The caustic regeneration reactor
was a 500cc round bottom flask equipped with a magnetic stirrer.
[0020] The filters for each reactor were medium porosity fritted glass immersion filters.
The filters were submerged in the corresponding slurry. Peristaltic pumps were used
to pump the liquids berween the slurry reactors. The pumps were activated by the pH
controller.
[0021] The regeneration reactor was charged with 25g CaO and 250cc 0.11 M Na
2SO
4. The resulting slurry was magnetically stirred at ambient temperature, isolated from
the atmosphere.
[0022] The leaching reactor was chafed with 700cc of 0.11 M Na
2S0
4 solution and heated to the desired temperature. The desired gas was injected thrtugh
the gas inlet in the bottom of the reaction. Stirring was accomplished by mechanically
driven impeller.
[0023] 50g of -200 mesh Ohio No. 6 coal was added to the leaching reactor. The coal was
allowed 10-15 minutes to wet, then the pH controller was allowed to bring the coal
slurry to the working pH by feeding a liquid stream from the leaching reactor to the
regeneration reactor and returning an equal amount of regenerated NaOH solution to
the leaching reactor.
[0024] The rate of reaction was followed in two ways. Slurry samples were removed from the
leaching reactor and the collected coal was analyzed for total sulfur. In addition,
the rate was monitored by observing the amount of regenerated liquid which was recycled.
[0025] At the end of the reaction the remaining coal slurry was filtered and a filtrate
sample was collected. The coal was washed several times with water and air dried.
[0026] Table I below contains data on leaching rates obtained in this experiment. Percent
pyrite removal from the coal is shown for different values of pH and resident times.
*Starting Coal - 1.58% pyritic sulfur, 1.57% organic sulfur and 0.55% sulfate sulfur.
[0027] It will be sean from Table I that the rate of leaching declines rapidly after 24
hours at pH 4-8. From 50-80% of the final sulfur removal occurs in this first 24 hour
period.
EXAMPLE II
[0028] Another experiment to evaluate the effect of pH on the final coal product was conducted.
The starting coal was again -200 mesh Ohio No. 6 coal and the slurry concentration
was about 6% by weight. Basically the same procedures were used in this experiment
as outlined in Example I above except that a treatment period of 6 days was chosen
to attempt complete removal of the pyritic sulfur. The pH of the slurry was varied
but always maintained at a value below about 9. The temperature was kept at about
70°C to try to offset any decline in the leaching rate due to the lower reaction pH.
The flow rate of oxygen was 0.2SCFH and the initial sodium concentration was 0.22
moles/liter. Table II below sets forth data obtained in the experiment.
It will be seen from Table II above that the amount of calcium and sodium in the product
coal declines with decreasing pH. At a pH of 5 or less both sodium and calcium are
less than 0.1% of the product coal.
[0029] Proximate analysis of the product coals is shown in Table III below.
It will be seen from Table III above that the optimum pH range for leaching pyrite
from coal is 5-6. 84% of the pyritic sulfur is removed in 6 days. If additional credit
is taken for the organic sulfur removed, the equivalent of 92% pyritic sulfur removal
is achieved. The residual sulfate sulfur in the product following a water wash is
low, e.g. about 0.1%, in coals treated at a pH of 4-8. At pH 3, significantly more
water insoluble sulfate 0.5% remains. The BTU content on a moisture and ash free basis
(MAF) is unaffected by treatment.
[0030] Thus it will be seen that the present invention provides a novel process for removing
pyrite from coal by oxygen leaching in caustic medium, e.g. NaOH, under mild conditions
wherein lime is added to the reaction product in order to regenerate additional alkali
or caustic reagent for use in the process. Although the slurry should be maintained
at a pH less than about 8, the optimum pH for sulfur removal was found to be 5-6.
In Figure 2 there is shown the relationship between the pH and the percent pyritic
sulfur leached, the percent water insoluble sulfate sulfur and the percent (Ca+Na).
It is apparent from the graph that a pH of 5-6 also optimizes the characteristics
of the product coal. It has been found that for optimum results the temperature of
the coal slurry should be maintained at above about 70°C and that a residence time
of about 6 days is usually required for completion of the desulfurization reaction.